The present invention relates to a method for the blow molding of plastic containers with a thickened bead at the bottom thereof.
Blow molding has been developing into a highly commercially successful means for producing numerous articles. However, many types of containers, such as those having sharp contour transition portions located at the bottom thereof, and particularly containers having thickened beads at the bottom thereof like a typical one gallon paint can, have not been produced by blow molding to any commercially significant degree due to the problem of thinning of the plastic material at the lower corner extremeties.
In smaller containers, the thinning problem, in some circumstances, has not posed a problem with respect to the blow molding of containers which do not require thickened beads at the lower end thereof; but, the production of a solid uniform bead at the lower end thereof has not been possible in any simple and highly reliable manner. This problem of reliably forming a solid lower end bead is even more accute in the production of larger containers. This problem has been sufficiently significant that in many cases the shape of the container has been redesigned so as to have a shape that does not have sharp transition areas so as to minimize the thinning problem, while in other cases where reinforcements and/or beads are required, separate reinforcing components have been placed into the molds so as to become embedded in the finished container. However, the use of separate reinforcing parts greatly increases the cost to manufacture containers by blow molding, while in other instances commercial considerations make it undesirable to change the standard shape and/or size that has been long in use for a given product.
To deal with the problem of unduly thin and weakened corners, it has been proposed (U.S. Pat. No. 3,585,262) to prepinch the parison and preblow it with a low pressure medium in the range of about 1-20 psi prior to insertion of the parison into the mold, and then to finish-blowing the parison, after closing of the mold, with a high pressure medium on the order of about 100 to 130 psi. However, this technique has not fully solved the problem and has not been adapted to the manufacture of containers with beads at their lower end.
Prior art attempts to blow mold containers with unitary beads have generally utilized a compression step for formation of these beads. Furthermore, these methods have generally required the use of either multiple molds (see, e.g., U.S. Pat. No. 4,117,062, U.S. Pat. No. 3,843,005) or the extrusion of a parison having wall portions of increased thickness which can be pinched between telescopable mold wall portions. The latter technique is only suitable for use in the formation of side wall beads and not beads at the bottom wall of a container, while in both instances these techniques greatly increase the cost of producing containers by blow molding.